1. Field of the Invention
The present invention relates to a receiving apparatus especially for receiving a high-frequency wave which can receive a plurality of waves of different frequency bands.
2. Background of the Invention
A reception hazard called snow noise in which interference resulting from noise caused by heat generated in a television receiver or the like has been known. If a level of a signal input to a receiver is large, then snow noise is not so remarkable. If on the other hand the input signal level becomes small, this interference becomes remarkable, which results in an unsatisfactory picture. This results from a weak reception electric field, a large loss in a long transmission line such as a coaxial cable or the like. A generally well known method of improving the snow noise interference is to connect a high-frequency amplifier called as a booster to a reception antenna to supply a received signal amplified (or attenuated) by the booster to circuits at the succeeding stage.
Programs of Japanese television broadcasting using ground waves are broadcasted by using different frequency bands, e.g, a VHF low channel (ranging from 90 MHz to 108 MHz), a VHF high channel (ranging from 170 MHz to 222 MHZ), and a UHF channel (ranging from 470 MHz to 770 MHz).
Programs of a digital audio broadcasting (DAB) recently started in Europe are broadcasted by using different frequency bands, e.g., a frequency band substantially ranging from 170 MHz to 240 MHz and a frequency band substantially ranging from 1450 MHz to 1500 MHz.
When a plurality of transmitted waves of high frequencies transmitted in different frequency bands set at an interval are received, it is the best method to receive the transmitted waves by using antennas dedicated for respective reception frequency bands. However, when a car-mount receiving apparatus and the like are used, design and layout of a car sometimes makes it difficult to provide a plurality of antennas therein. Therefore, a method of receiving a plurality of waves of high-frequency signals using different frequency bands by using only one reception antenna to input the received signals to a receiver through a coaxial cable is generally known. In this case, it is considered that a high-frequency amplifier such as the above booster or the like is connected to an antenna.
Since a gain of the high-frequency amplifier is fixed, the above arrangement is effective when an electric-field intensity transmitted to the reception antenna is comparatively weak. However, in a region with a strong electric field, such as a region near a transmitting station or the like, a high-frequency amplifier generates a nonlinear distortion, which aggravates its reception state. In particular, if a DAB receiver employing, for modulation of a broadcasting wave, an orthogonal frequency division multiplexing (OFDM) system which is one kind of multicarrier modulation system is employed, the broadcasted wave is a multicarrier wave and hence, if a transmission line has nonlinear characteristics, deterioration of characteristics resulting from intermodulation occurs, which leads to degradation of the reception.
When an antenna for receiving a plurality of high-frequency signals whose frequency bands are separated at a considerable interval is employed, it is difficult to set high-frequency gains in the respective frequency bands equal to one another. Moreover, a transmission attenuation amount of a coaxial cable is expressed by a sum of attenuation resulting from high-frequency resistance of a conductor and attenuation amount resulting from high-frequency dielectric loss of an insulator, and a dielectric loss in a VHF band is only several percentage or smaller, which means that the attenuation resulting from the high-frequency resistance of the conductor influences the total attenuation amount. Since the attenuation resulting from the high-frequency resistance is substantially proportional in a square root of a frequency, the attenuation amount of the coaxial cable represents a f characteristic.
Therefore, even if high-frequency signals of different frequency bands with similar electric intensities are transmitted, levels of the signals input to the receiver are varied depending upon their frequencies.